1. Technical Field
The present invention relates to a memory apparatus, and more particularly, to a phase change memory apparatus.
2. Related Art
Generally, a phase change memory apparatus performs a write verify function that verifies whether or not data has been correctly stored in a memory cell. The write verify function is usually performed as follows. First, logic-high data or logic-low data are written in the memory cell and the data stored in the memory cell is read by a verify read function. When the written data and the read data do not conform to each other, then data are rewritten in the memory cell. Subsequently another verify read function, which reads the data stored in the memory cell, is performed. When the written data and the read data finally conform to each other, then the write verify function ends.
The write verify function is needed to be performed to store correct data in the memory cell so that the data can be assured as being the correct output when performing a normal read operation. The data stored in the memory cell can be output by using an amplification function of a sense amplifier that differentially amplifies a current that penetrates through the memory cell and a reference voltage.
FIG. 1 is a diagram showing distribution of data that are stored in the memory cell of a conventional phase change memory apparatus.
Referring now to FIG. 1, it can be appreciated that the logic high data are distributed at an upper side and the logic low data are distributed at a lower side based on a reference voltage ‘Vref0’. An area that distinguishes the difference between the logic high data and the logic low data is a sensing window. Whether the data stored in the memory cell is the logic high or not is discriminated by the sensing window as a sensing margin.
The conventional phase change memory apparatus appropriately controls the amount of current penetrating through the memory cell. That is, when performing the write verify function, the current level of an input signal ‘SAI’ of a sense amplifier secures the sensing window. However, the input signal ‘SAI’ of the sense amplifier can adversely influenced by fluctuation in the process/voltage/temperature (PVT), which causes problems in reducing the sensing window. As a result of the sensing window's uncontrolled reduction can lead to malfunctions in the phase change memory apparatus.